1. Field of Invention
This invention relates to systems and methods for calibration of a document scanning system.
2. Description of Related Art
Due to instability in the scanning system illumination process, calibration of a document scanning system to a reference background is needed to obtain repeatable output. The calibration of a scanning system enables the system to properly differentiate from white and black areas. Normally, the calibration is done with reference to a document plane because the document plane represents what the machine is going to see. Therefore, a reference surface is typically provided on the document plane, e.g., a platen glass, for a calibration strip or label.
In a typical coplanar system, the calibration of the document scanning system is normally done upon powering up the system. A coplanar system exists when there is a calibration reference plane on the same plane as the document plane. For example, when a white calibration strip or label is attached to the top surface of a document plane. In this example, the white calibration strip or label is used for determining a level of whiteness. During the calibration in a coplanar system, a scanning element, e.g., charged-coupled devices (CCD""s), is positioned under the white calibration strip. The CCDs in the scanning system is able to identify the whiteness in the calibration strip and would wash out areas in a document corresponding to the level of whiteness so that it comes out white. However, document scanning systems need to be periodically calibrated because of drifts in lamp intensity and line voltage.
A non-coplanar calibration system exists when the calibration reference plane is offset from a document plane. Due to size, shape, or other factors attributable to a scanning system, the calibration reference plane may be unable to be placed on the document plane, i.e., the platen glass. In a non-coplanar calibration system, one must account for, primarily the difference in distance from the lamp at the calibration plane verses the document plane, and the reflectance of light through the platen glass during a copying process since a certain amount of light that goes through the platen glass can be lost due to the offset of the calibration plane. In addition, a document scanning system that uses full length mirrors and/or lamps as part of the assembly to scan the original document may experience lamp fall-off as the document approaches the ends of the lamp. For an infinitely long illumination source, light incident on a plane is inversely proportional to the normal distance from the source to the plane.
Lamp fall-off represents an effect that occurs as you get closer to the ends of the lamp. Light energy falls-off at the end locations of the lamp and as a result shadows can appear on the printed copy of the scanned document. As you move the calibration plane away from the document plane, which is usually the same plane that the original document is placed on, the lamp fall-off is more pronounced.
The methods and systems of this invention provides a calibration plane that is offset from the document plane in an image data source system and the system compensates for lamp fall-off.
This invention separately provides systems and methods that use a single calibration correction factor to account for the shift from calibration plane to document plane.
It should be appreciated that the systems and methods according to the invention are applicable to a wide variety of image data source systems. Thus, while the systems and methods in accordance with the invention may be directed to the calibration of a digital scanning system, it should be recognized that any non-coplanar system may be calibrated in accordance with the invention. Further, it should be recognized that the systems and methods described herein can be used in conjunction with various other processes and systems.
In the various exemplary embodiments of the systems and methods according to this invention, an image data source system, e.g., a scanner, with a document plane, e.g., a platen glass of various shapes and sizes, is provided whereby various limitations in the positioning of the calibration reference surface may exist. For example, compactness of design of a scanning system may inhibit the positioning of the calibration reference surface on the platen glass. Alternatively, there may not be enough surface space for the calibration strip on the document plane. Additionally, the orientation of the platen glass may cause inaccuracies in calibration if a calibration strip is placed thereon. Therefore, if it is desirable or required that the calibration reference surface be placed on a different plane from the document plane, the non-coplanar calibration system and methods of this invention provides calibration wherein at least lamp intensity and lamp fall-off are accounted for.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of this invention.